During a person's effort, i.e., a level of activity above a baseline activity level at rest, certain physiological parameters become modified. The relative changes in these parameters can in some instances be correlated to the level of effort and give information about the metabolic demand. Rate-adaptive metabolic demand pacemakers monitor such a physiological parameter and process parameter changes, based on a relationship between the monitored parameter and the cardiac rate, to determine a cardiac pacing rate. As used herein, the term "pacing rate" and "cardiac pacing" should be understood to mean the frequency or rate at which electrical pulses are delivered to a suitable location of the heart to stimulate a contraction. For example, the calculated cardiac rate may be controlled by an algorithm defining the relationship between the sensed parameter and the corresponding activity level. Thus, the calculated cardiac rate is controlled corresponding to the activity level represented by the physiological parameter.
In most metabolic demand pacemakers, one of these parameters is selected and monitored. A measurement of the parameter is made periodically. There is then periodically determined an increase, decrease or an upholding of the cardiac rate based on any relative change in the measured parameter.
One problem with these devices is that the parameter may have a non-negligible physiological variability from one measurement to the next. This in turn may lead to an error of interpretation about the real activity of the patient. For example, if the selected physiological parameter is the minute ventilation (also known as minute volume), namely a parameter corresponding to the air volume being expired during one minute, this parameter will present erratic variations called "ventilation gaps" during a period of effort. These ventilation gaps correspond to changes in the breathing rhythm that cause a decrease of the minute ventilation measure during a few breathing cycles.
The direct consequence in a controlled system responsive to such a parameter is a momentary decrease of the cardiac rate. This occurs, however, while the patient is still active and perhaps increasingly active, and is detrimental. Imagine not breathing while running up a flight of stairs and swimming underwater. Of particular concern is the occurrence of ventilation gaps during an effort phase which result in a decrease in the cardiac rate.
There is thus a continuing need to provide improved control of a metabolic demand pacemaker so that non-negligible physiological variations in a monitored parameter do not result in an erroneous change in the pacing rate.